The present invention relates to cellular telephone systems, and particularly to the provision of a communication path between a mobile telephone and a cellular telephone call-switching system. A cellular telephone system has a system of xe2x80x9ccellsxe2x80x9d which are geographical areas, each of which is associated with a radio base station. As the mobile unit moves from one cell to another, radio contact is xe2x80x98handed-overxe2x80x99 from one radio base station to another.
As the usage of mobile telephones increases, the provision of acceptable quality of service to subscribers becomes increasingly difficult as the demand for radio channels exceeds the availability of channels allocated to network operators. The allocation of radio frequency (r.f.) spectrum to different services is carried out by national governments which operate within the framework set by the WARC (World Administrative Radio Conference). Furthermore, expansion of the allocated band beyond a certain size would cause technical problems for the design of mobile telephones. In particular, there would be the problem of providing effective broadband frequency synrthesizers, r.f. amplifiers and antennas.
It is an aim of the present invention to overcome the aforementioned problem by providing an additional communication path independent of the system of cells of the cellular telephone system. It is known to provide a mobile telephone which is also capable of operation according to a cordless radio standard (e.g. DECT) to a nearby radio base station, or to a fixed (PSTN) line dedicated to the user or a specified group of users. Examples of the latter are to be found in IBM Technical Disclosure Vol 38 No 6 and European patent Application EP0660628 (Nokia), both published in 1995. However, such systems require a separate user subscription on the fixed network, and an associated separate network identity (telephone number), requiring call diversion arrangements to be set up to allow the user to be contacted. The cordless examples also require their own allocation of radio frequency spectrum, and so they do not overcome the fundamental problem of limited radio spectrum.
According to a first aspect of the invention, there is provided a cellular telephone system comprising an additional communication path independent of the system of cells of the cellular telephone system, between a mobile telephone and a cellular telephone call-switching system, the communication path comprising an interface means for providing a communications connection between a mobile telephone and the cellular telephone call-switching system so as to by-pass the telephone""s radio antenna.
In another aspect, the invention comprises an interface apparatus for providing a communications connection between a mobile telephone and a cellular telephone call-switching system so as to by-pass the telephone""s radio antenna, the apparatus comprising a first connector for exchanging speech and data signals with a cellular mobile telephone, a second connector for connecting the apparatus to a transmission line, and means for exchanging said speech and data signals via the second connector.
In a further aspect, the invention comprises a telephone for a cellular telephone system including processing circuitry and an externally accessible connector, the connector being coupled to the processing circuitry to exchange baseband speech and data signals therewith, and having radio frequency transceiver circuitry for coupling to the processing circuitry to exchange baseband speech and data signals therewith, the processing circuitry including means for detecting whether the externally accessible connector is in communication with a complementary connector, and switch means under the control of the processing circuitry to disconnect the processing circuitry from the transceiver circuitry when the externally accessible connector is so connected wherein the first connector communicates with the mobile telephone by means of art electrical connection, further comprising a power supply connection for supplying electrical power to the mobile telephone through the electrical connection.
Preferably, such a telephone includes r.f. circuitry for transmitting and receiving speech and data signals, and a further externally accessible connector and switching means, wherein the switching means is responsive to a signal applied to the further connector to disable the r.f. circuitry.
In a further aspect, the invention comprises a method of connecting a mobile telephone to the call-switching system of a cellular telephone system comprising the step of establishing an additional communication path, independent of the system of cells of the cellular telephone system, between the mobile telephone and a fixed part of the cellular telephone system, by putting the mobile telephone in communication with an interface means, the interface means being connected to the call-switching system of the cellular telephone system by a communications path.
The interface apparatus may provide a communications connection to the mobile telephone for baseband speech and control data signals. In this case, the interface means may comprise a modem for transmitting the speech and control data over longer distances. The modem may include dialling means so that a switched network, e.g. PSTN (public switched telephone network) or ISDN (integrated services digital network), may be used for the communications path. Alternatively, the communications path may comprise a fixed point-to-point signal path. This could be a leased line, a permanent ISDN connection, a cable television (CATV) transmission line or a microwave link.
Advantageously, the interface means is arranged for connecting a plurality of mobile telephones to the call switching system and the bandwidth of the communications path is sufficient for a plurality of simultaneous calls.
In order for the present invention to be implemented with existing mobile telephone equipment, the interface means may provide a connection with the r.f. circuitry of a mobile telephone, for carrying r.f. speech and data signals. Some handheld telephones are already provided with an r.f. connector to enable them to be used with adaptors for vehicle-mounted operation.
Preferably, the communication path includes a transmission line and the interface means comprises means for connecting the telephone""s r.f. circuitry to the transmission line. More preferably, the communication path comprises a point-to-point microwave link and transposer means for transposing the signals on the transmission line to the operating band of the microwave link and transposing signals received from the microwave link to the operating band of the mobile telephone and applying them to the transmission line. Thus, the telephone and a BTS (base transceiver site) substitute, providing connection to the mobile telephone call switching system, may both operate with substantially unchanged software. In practice, only the BTS software would need to be changed and then primarily only in respect of the control of its frequency synthesizers. However, even this change could be avoided if a transposer means were to be used at the BTS. If the xe2x80x9cBTSxe2x80x9d is connected directly to the transmission line, the transposer means can be dispensed with.
Advantageously, the interface means provides a communications connection to the transmission line from the r.f. circuitry of a plurality of mobile telephones.
Preferably, the interface means is provided with user input means to enable the telephone or ISDN (Integrated Services Digital Network) number of a BTS to be entered. In the simplest case, this could merely be a numerical key pad. However, the service provider may wish to keep the telephone or ISDN numbers of its BTSs secret. In this case, the interface means could be programmed with a table of encrypted numbers. The correct number could then be retrieved and decrypted on the basis of a postcode or zip code entered by a user. A further option would be for the user to be instructed to call a central station using his mobile telephone. The cellular system can identify the location of the mobile telephone in the cellular network and can then download the correct BTS telephone number or ISDN number to the mobile telephone. Connecting the mobile telephone to the interface means for the first time would cause the mobile telephone to transfer the downloaded number to the interface means.
In a multi-user embodiment, the interface apparatus may comprise one or more further communications connectors for receiving baseband speech and data signals from one or more further cellular mobile telephones, and a multiplexer for multiplexing signals from the first and further connectors, the modem being arranged for transmitting the output of the multiplexer.
Preferably, the interface apparatus includes control means and dialling means, wherein the control means is responsive to cellular telephone data signals from the first connector (and further connectors, if provided) to cause the dialling means to output telephone or ISDN dialling signals via the second connector.
In most of the embodiments to be described below, a mechanical/electrical plug and socket connection is used to provide the communications connection between the telephone and interface unit, which may also provide a power supply to the telephone. However, connection may instead be provided by means of an optical (e.g. infra-red) or ultrasonic beam, carrying either a digital signal (e.g. according to the IrDA (Infra-red data access) standard, originally developed for use in connecting portable computers to printers etc), or an analogue signal.